The present invention relates to a frost control system for self-contained air conditioning units. Typically, the units are adapted to be mounted in the outer wall of an enclosure to be conditioned and comprise a housing which is divided into an indoor section and an outdoor section and utilized for heating the air from the enclosure during the winter and cooling the air from the enclosure during the summer. An indoor heat exchanger is disposed in the indoor section while an outdoor heat exchanger and usually the compressor are located in the outdoor section. The compressor is reversibly connected to the heat exchangers so that the indoor heat exchanger functions as an evaporator when the unit is operating in the cooling cycle and the outdoor heat exchanger functions as the evaporator in the heating cycle. Suitable independent fan means are provided for circulating indoor air over the indoor heat exchanger and outdoor air over the outdoor heat exchanger during operation of the system in either the heating or cooling cycle.
The control system of the present invention can be used in reversible type heat pumps or in a refrigeration system having a unidirectional refrigerant flow wherein the condenser and evaporator retain their functions, but the air directed across them is redirected for different operations. This type is generally known as a reverse air cycle heat pump and, in the cooling mode, outdoor air is passed through the condenser and indoor air is passed through the evaporator. Conversely, in the heating mode, outdoor air is passed through the evaporator and indoor air is passed through the condenser.
In either type heat pump, under certain operating conditions with the unit in the heating cycle, the heat exchanger functioning as the evaporator is subjected to outside air and may operate at ambient temperature and conditions that will cause the accumulation of a coating or layer of frost thereon. Since a frost layer operates as a barrier to heat transfer between the evaporator and the air being circulated therethrough, the efficiency of the unit is markedly reduced. Also, unless means are provided for interrupting this accumulation of frost, the evaporator surface can become completely covered with a layer of frost which may eventually cause motor failure or other damage to the unit.
In many prior art systems, frost control means operable in the heating cycle are provided wherein a frost sensing element is employed to detect the presence of frost on the heat exchanger exposed to outdoor ambient and functioning as an evaporator. In some control systems employed in reverse cycle heat pumps, the presence of frost on the heat exchanger exposed to outdoor ambient causes the refrigerant system flow to reverse and the frost laden heat exchanger allowed to function as the system condenser until the frost is eliminated. At the same time, in some control systems, the heating process continues by energizing an electric resistance heater so that warm air is circulated into the enclosure being heated. In other prior art frost control systems employed in either the reverse refrigerant or air cycle heat pump, the presence of frost on the heat exchanger exposed to outdoor ambient causes the deenergization of the compressor, thereby interrupting the heating cycle until the layer of frost is eliminated. In this instance, an electric resistance heater may be energized to supply heat to the enclosure while the frost is being disposed of.
In both types of defrost systems, it is necessary that the heating process be interrupted while the frost layer is disposed of. Accordingly, by the present invention a control is provided to eliminate frost when present under certain ambient conditions without interrupting the normal cycling of the unit in the heating cycle.